A solar cell which converts photoelectrically solar energy into electrical energy is a promising power generator. Currently, however, the costs of semiconductor materials to make a solar cell and the costs of making a solar cell are so high that the unit cost (i.e., unit energy cost) to generate 1 kW of electric power using a solar cell is approximately several times as high as when electric power is generated using a thermal power system.
Also, the sun moves from the east to the west. That is why if the photosensitive plane of a solar cell is fixed, the angle of incidence of sunlight onto the solar cell changes as the sun moves. And as the angle of incidence changes, not only the effective light receiving area (which is proportional to cos θ if the angle formed between a normal to the photosensitive plane of a solar cell and the incoming light ray is θ) but also the reflectance from the photosensitive plane of the solar cell that would cause Fresnel reflection change as well. In general, at around noon, the absolute value of θ becomes the smallest, the effective light receiving area becomes the maximum, the surface reflection loss becomes the minimum, and the quantity of light received becomes the maximum.
For these reasons, some people have proposed a system for making a solar cell track the sun so that the incoming solar ray is always incident perpendicularly onto the photosensitive plane of the solar cell by shifting the entire solar cell according to the azimuth of the sun while concentrating the incoming solar ray with a large cross-sectional area onto the solar cell with a small area through a lens. Such a system is called a “sun-tracking concentrating solar cell”. To track the sun, however, an apparatus for detecting the azimuth of the sun, a driver for rotating the solar cell in its entirety, and other devices are needed, thus raising the unit energy cost of such a sun-tracking concentrating solar cell. That is why the overall unit energy cost should be decreased either by running a huge number of solar cells using a single detector or driver with a lot of lens-solar cell combinations arranged as an array or by adopting an optical system that would achieve high light concentration efficiency (i.e., the ratio of the area of a lens to the area of a solar cell).
The structure of a conventional concentrating lens array for use in a sun-tracking concentrating solar cell will be described with reference to FIG. 14, which schematically illustrates a cross-sectional structure of a solar cell and a concentrating lens array arranged on the cell. As shown in FIG. 14, a plurality of photosensitive sections 3 are arranged as an array on a solar cell substrate 100, which includes a cooling mechanism (not shown) 10 prevent the incoming sunlight being concentrated onto the photosensitive sections 3 from overheating the solar cell substrate 100 to more than a predetermined temperature.
A concentrating lens array 5 is arranged over the solar cell substrate 100. The concentrating lens array 5 includes a plurality of micro lenses 5a which are arranged as an array in order to concentrate the incoming sun light onto the respective photosensitive sections 3. The surface 5s of each of those micro lenses 5a has an approximately spherical shape. An incoming solar ray 4 which is incident on a micro lens 5a along its center axis L gets refracted at its surface 5s to turn into a light ray 4a that converges toward a point F on its associated photosensitive section 3. This light ray 4a is received at the photosensitive section 3 and converted photoelectrically. In this structure, the lens array forms an integral part of the photosensitive sections of this solar cell, and an inexpensive solar cell with high light concentration efficiency would be provided by having this solar cell face the sunlight radiating direction.